The NASA objective of expanding the human experience into the far reaches of space will require the development of regenerable life support systems. A key element of these systems is a means for solid waste resource recovery. The objective of this invention is to demonstrate the feasibility of pyrolysis processing as a method for the conversion of solid waste materials in a Controlled Ecological Life Support System (CELSS). A pyrolysis process will be useful to NASA in at least four respects: 1) it can be used as a pretreatment for a combustion process; 2) it can be used as a more efficient means of utilizing oxygen and recycling carbon and nitrogen; 3) it can be used to supply fuel gases to fuel cells for power generation; and 4) it can be used as the basis for the production of chemicals and materials in space.
A key element of a CELSS is a means for solid waste resource recovery. Solid wastes will include inedible plant biomass (IPB), paper, plastic, cardboard, waste water concentrates, urine concentrates, feces, etc. It would be desirable to recover usable constituents such as CO2, H2O, hydrogen, nitrogen, nitrogen compounds, and solid inorganics. Any unusable byproducts should be chemically and biologically stable and require minimal amounts of storage volume.
Many different processes have been considered for dealing with these wastes: incineration, aerobic and anaerobic biodigestion, wet oxidation, supercritical water oxidation, steam reforming, electrochemical oxidation and catalytic oxidation. However, some of these approaches have disadvantages which have prevented their adoption. For example, incineration utilizes a valuable resource, oxygen, and produces undesirable byproducts such as oxides of sulfur and nitrogen. Incineration also will immediately convert all of the waste carbon to CO2, which will require storing excess CO2.
“Pyrolysis,” in the context of this application, is defined as thermal decomposition in an oxygen-free environment. Primary pyrolysis reactions are those which occur in the initial stages of thermal decomposition, while secondary pyrolysis reactions are those which occur upon further heat treatment. A pyrolysis based process has several advantages when compared to other possible approaches for solid waste resource recovery: 1) it can be used for all types of solid products and can be more easily adapted to changes in feedstock composition than alternative approaches; 2) the technology is relatively simple and can be made compact and lightweight and thus is amenable to spacecraft operations; 3) it can be conducted as a batch, low pressure process, with minimal requirements for feedstock preprocessing; 4) it can produce several usable products from solid waste streams (e.g., CO2, CO, H2O, H2, NH3, CH4, etc.); 5) the technology can be designed to produce minimal amounts of unusable byproducts; 6) it can produce potentially valuable chemicals and chemical feedstocks (e.g., nitrogen rich compounds for fertilizers, monomers, hydrocarbons); and 7) pyrolysis will significantly reduce the storage volume of the waste materials while important elements such as carbon and nitrogen can be efficiently stored in the form of pyrolysis char and later recovered by gasification or incineration when needed. In addition to being used as the primary waste treatment method, pyrolysis can also be used as a pretreatment for more conventional techniques, such as incineration or gasification.
The primary disadvantages of pyrolysis processing are: 1) the product stream is more complex than for many of the alternative treatments; and 2) the product gases cannot be vented directly in the cabin without further treatment because of the high CO concentrations. The former issue is a feature of pyrolysis processing (and also a potential benefit, as discussed above). The latter issue can be addressed by utilization of a water gas shift reactor or by introducing the product gases into an incinerator or high temperature fuel cell.